New option to only work with subscribed folders

Patch from Sean Finney, slightly modified by John Goerzen to change
name of config file option

fixes deb#499588

offlineimap.conf

@@ -212,6 +212,10 @@ type = Maildir
 
 localfolders = ~/Test
 
+# Specify whether to process all mail folders on the server, or only
+# those listed as "subscribed".
+subscribedonly = no
+
 # You can specify the "path separator character" used for your Maildir
 # folders.  This is inserted in-between the components of the tree.
 # It defaults to ".".  If you want your Maildir folders to be nested,